Fender 6G16 Brownface Vibroverb - reverb mods

ampman73

Member
Messages
5
Hi guys,

Just wondering if there was a way of making the Brownface Vibroverb 6G16 have more of a blackface splashy type reverb?

It's a nice reverb, but a bit dark sounding for me. I've compared schematics and whilst there's a few component differences and it uses a 12ax7, nothing jumps out at me as to what makes it so much darker.

Thanks!
 

pdf64

Member
Messages
8,912
The grid circuit impedance of the recovery stage is a bit higher for the 6G16 than its AB763 version.
Hence its Miller capacitance roll off will kick in at a lower frequency.
http://schems.com/schematicheaven.net/manu/fender/vibroverb_6g16_schem.pdf
http://schems.com/schematicheaven.net/manu/fender/vibroverb_ab763_schem.pdf

But I'm not sure that this effect would be significant.
What you're hearing may just be a bad part
Have you tried a swapping around tubes and tanks, checking for resistor value drift?
 

ampman73

Member
Messages
5
The grid circuit impedance of the recovery stage is a bit higher for the 6G16 than its AB763 version.
Hence its Miller capacitance roll off will kick in at a lower frequency.
http://schems.com/schematicheaven.net/manu/fender/vibroverb_6g16_schem.pdf
http://schems.com/schematicheaven.net/manu/fender/vibroverb_ab763_schem.pdf

But I'm not sure that this effect would be significant.
What you're hearing may just be a bad part
Have you tried a swapping around tubes and tanks, checking for resistor value drift?

Hi pdf64,

Thanks for your reply. This 'darker sounding reverb' is actually a characteristic of these amps that players love! I just wondered if there was a way to brighten the reverb up to make it more blackface-esq...
 
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pdf64

Member
Messages
8,912
You could try tacking a 470k resistor in parallel with the existing 470k resistor, between the reverb control wiper and the V4 pin 7.
Another difference is that the 6G16 has a lot more negative feedback, which will act to negate the treble boost caused by the interaction between a tube amp's high (open loop) output impedance and speaker inductance.
But that will affect the dry signal too.
 

HotBluePlates

Member
Messages
14,050
... Another difference is that the 6G16 has a lot more negative feedback ...

Does it? I was seeing 10kΩ against 4.7kΩ as being a touch less NFB than 820Ω against 47Ω.

EDIT: Silly me should have written it on paper; as you say, much more feedback.
 
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Messages
6,602
Hot Blue Plates, for some reason I have a schematic for a 6G16 that I have a correction marked on that tail resistor. I have 470ohms written in there where the schematic calls for a 4.7K. I did this 'correction' long ago. And...I just looked at the schematic for the 6G16 RI, and they show a 10K top resistor with a 470 ohm tail resistor. Somewhere I must have read that the 4700 ohm was an error on the schematic. ???? Any insight? With a 10K on top of the 470 ohm resistor, that puts that NFB circuit more nearly in the same ballpark as the 820 ohm top resistor and the 100 ohm tail resistor, doesn't it?
 

HotBluePlates

Member
Messages
14,050
10kΩ:4.7kΩ = 100:47 (Schematic values)
10kΩ:470Ω = 1,000:47 (Correction values)
820Ω:47Ω = 820:47 (Typical blackface values)

My mistake earlier. The schematic values in the 6G16 indeed indicate much more feedback. The correction values land at a value more like what I was thinking before writing it down, with a touch less feedback than the 820Ω against 47Ω.
 

JJman

Member
Messages
994
I would try a cap across the 470k that goes to the wiper of the reverb control. Maybe 1000pf to start for experimenting.
 

wyatt

Member
Messages
4,171
10kΩ:4.7kΩ = 100:47 (Schematic values)
10kΩ:470Ω = 1,000:47 (Correction values)
820Ω:47Ω = 820:47 (Typical blackface values)

My mistake earlier. The schematic values in the 6G16 indeed indicate much more feedback. The correction values land at a value more like what I was thinking before writing it down, with a touch less feedback than the 820Ω against 47Ω.

Except the 6G16 used a 4-ohm OT (2x10) and the AB763 Vibroverb used an 8-ohm OT (1x15). The 6G16 NFB loop is starting off with less voltage (-25%? -30%) at the tap.

The non-8-ohm Blackface amps....BFVR, SFSR, BFTR, etc....used 820R:100R.
 
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66 Kicks

Member
Messages
137
You could also try a .002uF capacitor in parallel with the 220K resistor on the reverb output. A lot of people think this capacitor cuts highs off of the reverb output, but it actually does the opposite.
 

pdf64

Member
Messages
8,912
The 6G16 NFB loop is starting off with less voltage (-25%? -30%) at the tap
P=(V*V)/R, so 1 watt across 16 ohms requires 4V, across 8 ohms would require 2.828, across 4 ohms would require 2V.
The relationship is that electronics favourite root2, so 1.414 going up, 1/root2 = 0.7071 going down.
 

HotBluePlates

Member
Messages
14,050
Except the 6G16 used a 4-ohm OT (2x10) and the AB763 Vibroverb used an 8-ohm OT (1x15). The 6G16 NFB loop is starting off with less voltage (-25%? -30%) at the tap.

The non-8-ohm Blackface amps....BFVR, SFSR, BFTR, etc....used 820R:100R.

See... It's time for me to stop & think before posting! And take a closer look at the schematics! :confused::rolleyes:o_O

Oh well... I'll do better next time! :cool:
 

JPH118

Silver Supporting Member
Messages
4,764
After delving into this circuit a few months back, with the help of some of the great minds here, I determined it was mainly the tapped-treble pot of the 6G16 along with the 3.3M/10pf reverb mixing network of the AB763 that mostly set the two circuits apart, resulting in the warmer/less verb of the 6G16 versus the brighter/more verb of the AB763. That piggybacked mixing network holds the dry signal back but keeps it bright, and the balance of reverb in the overall mix can be much greater than the 6G16, resulting in the splash you mentioned... i think this might be what you're looking for?? I can't math like these other great minds can, but I love to experiment and listen.
 




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